Scientists find new threat to Hawai'i's tomato crop

Scientists at the University of Hawai‘i at Mānoa’s College of Tropical Agriculture and Human Resources (CTAHR) report that a destructive viral disease affecting tomatoes has found its way to the islands, posing a threat to the state’s $10-million-per-year tomato industry. Tomato yellow leaf curl, caused by the Tomato Yellow Leaf Curl Virus (TYLCV), was first discovered on Maui and O‘ahu in November 2009.

Although TYLCV is new to Hawai‘i, it is present in many regions where tomato is grown. The disease is particularly destructive in tropical and subtropical regions, where it has caused total crop loss in tomatoes. It is not known how the virus entered Hawai‘i.

Symptoms of tomato yellow leaf curl

The new growth of plants with tomato yellow leaf curl has reduced internodes, giving the plant a stunted appearance. The new leaves are also greatly reduced in size and wrinkled, are yellowed between the veins, and have margins that curl upward, giving them a cup-like appearance. Flowers may appear but usually will drop before fruit is set.

Spread of TYLCV

TYLCV is primarily transmitted by the sweetpotato whitefly (Bemisia tabaci) and the biotype B (or silverleaf) whitefly (Bemisia argentifolii). These whiteflies can acquire the virus in as little as 5 minutes by feeding on infected plants, and they remain infective for life; the virus, however, is not passed on to their progeny. TYLCV is not spread by other whitefly species such as the greenhouse whitefly, Trialeurodes vaporariorum, which is also common in Hawai‘i. TYLCV cannot be spread by seed and is not mechanically transmitted (e.g., by pruning equipment or by touch). Long-distance spread of TYLCV occurs primarily by movement of infected plant material or by wind dispersal of whiteflies harboring the virus.

Other hosts of TYLCV

TYLCV infects a wide range of plant crops and weeds, but it usually does not cause symptoms in these hosts. Most solanaceous plants such as tomato, eggplant, potato, tobacco, and pepper can be infected with TYLCV but remain healthy in appearance. Common bean is also a host and will sometimes display leaf curl symptoms when infected. Many common weeds are also host to the virus and may or may not develop symptoms when infected.

Management of tomato yellow leaf curl

Symptomatic plants should immediately be carefully removed, bagged, and discarded to prevent the spread of whiteflies on them that may be carrying the virus. Remove weeds and avoid growing other solanaceous crops near tomato plants whenever possible, as these plants may serve as a reservoir of the virus. UV-reflective mulch can also be effective at deterring whiteflies from landing on crops.

Insect exclusion screens rated for whitefly can be used for individual plants and are now being used for screenhouse construction in regions where TYLCV is prevalent.

It is important to visually monitor the crop for whitefly infestations and apply insecticides to suppress whitefly populations and reduce the spread of TYLCV. Imidacloprid-based systemic insecticides (e.g., Admire® Pro, Provado®) are effective for both adult and nymph stages but should not be used in soil-free media as a drench. Pyrethrins and pyrethroid-based insecticides effectively control adult whiteflies. Spirotetramat (Movento), with methylated seed oil plus silicone surfactant, and insecticidal oils, soaps, and other extracts are effective at suppressing all stages. It is important to alternate insecticide chemical types to prevent the build-up of pesticide-resistant insect populations. Always read and follow the label instructions of any insecticide before its application.

Many TYLCV-resistant tomato varieties are available for both field and greenhouse production systems. These varieties come in many fruit shapes (globe, Roma, cherry, etc.) and as both determinate and indeterminate plant types. Under stress, however, these resistant varieties can lose their protection and develop symptoms of tomato yellow leaf curl. As such, it is important to keep plants healthy by proper irrigation and fertilizer regimes and to keep them free of other pests and diseases.

For further information on tomato yellow leaf curl disease, see http://www.ctahr.hawaii.edu/oc/freepubs/pdf/PD-70.pdf . Farmers with any of these symptoms in their tomato crops should contact their local Cooperative Extension Service office or the CTAHR Agricultural Diagnostic Service Center at 956-6706.